Nontyphoidal Salmonella (NTS) infections, including Salmonella typhimurium (Stym), are an important health concern worldwide. Stym can cause gastroenteritis in people of all ages and severe invasive diseases in infants and immunocompromised individuals. Studies suggest the importance of a variety of immunological factors including the Natural resistance-associate macrophage protein 1 (Nramp1), Toll-like Receptor (TLR) -4/MD-2 complex, and the interleukin (IL)-12/IL-23-interferon (IFN)-? axis. Following recognition of Stym by antigen presenting cells (APCs) via TLRs cytokines are produced to contain the infection during the first days and then promote the development of adaptive, antigen-specific cellular immunity to the invading microbe, which must take place in a timely manner and be of proper magnitude and quality to eradicate or control infection. Th1 CD4 T cell responses have been shown to be crucial for protection against Stym in mice, but more recent evidence both from humans and mice with defects affecting the IL-12/IL-23 axis suggests that IL-23 and Th17cells may also contribute. To address these possibilities, Aim 1 will investigate whether production of IL-23 in addition to IL-12 by antigen-presenting cells, and Th17in addition to Th1 cytokines by T cells, are important for protection against Stym. We hypothesize that the joint action of Th1and Th17cells rather than either of them alone will provide the host with the most robust protection against Stym. TLR4/MD-2 LPS receptor complex in defense against NTS is evident from the increased susceptibility of TLR4-deficient mice to infection with Stym. TLR4 signaling is further supported by the observation that upon entry into phagocytes, Stym alters its LPS from a solely hexa-acylated molecule, which is a strong agonist for human and mouse TLR4/MD-2, to a mixture of less agonistic structures, particularly for human TLR4/MD-2. To test this immune evasion strategy as it would occur in humans, we have developed mice that express human TLR4/MD-2 rather than mouse TLR4/MD-2. We will use these mice in Aim2 to determine how the sensory function of TLR4/MD-2 is affected by the modulation of Stym's LPS structure in vivo. We hypothesize that humanized TLR4/MD-2 mice will be more susceptible than WT mice and exhibit diminished and/or qualitatively different CD4 T cell responses to Stym. Salmonella, a worldwide health concern, can cause gastroenteritis in people of all ages and severe invasive diseases in infants and immunocompromised individuals. Our studies seek to provide new insights into the immune mechanisms that protect against NTS, identify differences that contribute to greater vulnerability, and suggest strategies by which to overcome this vulnerability.